Heat-expandable microcapsules comprising thermoplastic polymer shell in which low-boiling-point solvent is encapsulated have been under investigation for a long time. For example, the technique disclosed in Japanese Patent Publication Sho-42-26524 relates to a general production process of heat-expandable microcapsules. The technique disclosed in Japanese Patent Laid Open Sho-62-286534 and Japanese Patent Laid Open Hei-5-285376 (U.S. Pat. No. 5,536,756) provides the processes for producing heat-resistant heat-expandable microcapsules wherein the shell of heat-expandable microcapsules is formed by polymerizing the monomers containing 80% or more of acrylonitrile or the monomers which form homopolymers of high glass-transition point for the purpose of increasing the starting temperature of microcapsules' expansion and by adding a small amount of radically polymerizing polyfunctional monomers to the above monomers for the purpose of improving the heat-resistance of resultant microcapsules with the polyfunctional monomers which thermally polymerize and harden the shell of the microcapsules at their expansion. Those processes do not work well as expected if the polymers are not cross-linked spontaneously and densely when the microcapsules are heated.
Recently the character of synthetic polymers has been greatly changed since engineering plastics were developed and Kaminsky's catalyst was invented. In addition, the needs for recycling and reuse of polymers have emerged with the change of the social concept toward environmental problems. And crystalline polymers such as polyethylene, polypropylene and PET are used frequently instead of cross-linking polymers such as polyurethane.
Under such situation, new expanding agents, which expand at higher temperature than conventional ones, are required. In addition, they are required to have heat-resistance durable enough during long-time use at high temperature. Organic expanding agents meet those requirements but they are apt to form a continuous cell, which has poor property such as low tenacity of polymer. The inventors have produced microcapsules that are durable under high-temperature molding and expand to produce single closed cells when required.
The microcapsules can also be applied to various fields as a filler of low density and high heat resistance when the microcapsules are expanded and processed into composite with inorganic components for controlling their specific gravity.
The expanding temperature of conventional expandable microcapsules having polymer shell of which main component is acryl ester depends on the Tg and molecular weight of the polymer and on the boiling point of encapsulated liquid. For increasing the starting temperature of microcapsules' expansion, a liquid of higher molecular weight needs to be encapsulated and thus the quantity of the encapsulated liquid must be increased to keep the internal pressure of vaporized liquid, which decreases with the increase of molecular weight of an encapsulated liquid, at a proper level. In this case, the shell of microcapsules becomes thinner due to the decrease of the ratio of polymer that forms the shell of microcapsules. At the initial step of expansion, such thin shell cannot retain encapsulated liquid (gas of vaporized encapsulated liquid) in microcapsules against the high pressure of the vaporized liquid and the gas is exhausted drastically to decrease the expansion ratio. In addition, the maximum expansion ratio is apt to be lowered.
Drastically softened polymer shell cannot retain gas in microcapsules and results in the leakage of gas through shell. Polymer shell of higher softening point (or higher Tg) is desirable for microcapsules durable at high temperature.
Many of the acryl ester polymers having high Tg have bulky structure. And the polymers of which molecular chains are not thermally kinetic due to steric hindrance have high Tg. Thus their molecules have wide interspace and do not soften easily owing to their bulky structure.
Such property is preferable to the softening of polymer shell that results in the dissolution and release of the molecules of encapsulated liquid, but it is disadvantageous for producing heat-resistant microcapsules (which retain the gas of encapsulated liquid within polymer shell).